Sea grasses, the leading indicator of the health of Tampa Bay, have now expanded to cover more of the bay than at any time since the 1950s.

And part of the reason for the good news may lie with the region's severe economic slump.

Tampa Bay gained 3,250 acres of sea grass between 2008 and 2010 — an 11 percent increase that is the largest two-year expansion of sea grasses since scientists began regular surveys, according to a report released Friday by the Tampa Bay Estuary Program.

That means the bay now supports 32,897 acres of sea grasses, which feed manatees and other marine life as well as providing shelter and nursery areas for sportfish such as sea trout, snook and redfish.

As a result "we've seen an overall rebound in the fish population," said Nanette O'Hara, the estuary program's outreach coordinator and an avid angler. Redfish and spotted sea trout in particular are booming, she said, and even tarpon have come back.

"Tarpon were once our signature game fish in the bay," O'Hara said. "There was a Tampa tarpon tournament every year starting in the 1950s, but they had to stop it in the late '70s or early '80s because there weren't any tarpon around any more. But now the tarpon have made a big comeback in the bay."

All major bay segments showed gains in sea grass growth, according to the report — even the Old Tampa Bay segment in the northern part of the bay, which has been plagued by algae blooms and an expanding layer of thick, soupy muck near Safety Harbor in recent years.

Why such a vast improvement? For one thing, the Tampa Bay area has suffered through several years of comparatively low rainfall, O'Hara said. That meant less runoff polluted by front-lawn fertilizer was washing into the bay — something scientists jokingly refer to as "urban slobber."

The other factor may be the slowdown in the region's growth since the mortgage meltdown hit, she said. That has reduced the number of new homes and commercial properties being built, which also reduces the pollution load.

And since air pollution produces 30 percent of the nitrogen that fouls the bay, the recession has helped by cutting back on the number of cars and trucks being driven around the region, she said.

It's still too early to credit any of the improvements to the recent enactment of fertilizer restrictions by local governments, she said, but the new rules may produce even better water quality in the next report.

Despite the good news, Tampa Bay still has 5,103 acres to go to reach the target goal of 38,000 acres of sea grass set by the estuary program, an independent body created by Congress in 1991 to focus on cleaning up the bay.

Tampa Bay is Florida's largest open-water estuary, covering about 398 square miles at high tide. In the 1950s and '60s, dredging created land for development around the bay but wiped out much of its sea grass, hurting commercial and recreational fishing. Polluted runoff killed even more sea grass. By the early '90s, the bay had lost 80 percent of its sea grass, more than anywhere else in Florida.

Changes in local sewer plant operations started the cleanup. Then the region's utilities cut the amount of nitrogen oxide pollution pouring out of their power plant smokestacks.

The data on sea grass growth in the bay was collected by scientists with the Southwest Florida Water Management District. The results of their studies have been used to track trends in sea grass extent in estuaries throughout Southwest Florida since 1988.